Installation of underground electric transmission and distribution systems, including those for windfarms, involves digging a trench, laying cable, pipe or conduit (through which cable will be housed), and then backfilling the trench. The backfill surrounding a cable, pipe or conduit must provide a low thermal resistivity (usually less than 100° C.-cm/W at 0% moisture), and must have enough strength to support loading at the surface.
Thermal resistivity relates to the ability of a material to conduct heat. Thermal resistivity is a measure of the ability of a material to impede the flow of heat.
Thermal resistivity (Rho) is typically given in units of (° C.)(T)/W, where ° C. is temperature, T equals a thickness of a material, and W equals the power flowing through the material in Watts. The lower the thermal resistivity of a material, the greater the ability of a material to absorb and transfer heat.
Underground electrical and transmission systems can generated substantial amounts of heat. Heat generated from such underground electrical cables must dissipate quickly and efficiently to allow the cable to operate at or above its design ampacity. If this heat is not dissipated quickly and efficiently, then the amount of current a line can safely carry (ampacity) is substantially reduced. If a line gets overheated, then the line can prematurely fail or a backfire can start.
All heat generated by underground power cables needs to be dissipated through the soil. Underground electrical systems are often designed to last 30 years, and designed, in part, based on the thermal resistivity of the soil. Selecting an good thermal backfill becomes very important for several reasons. Heat generated from a power cable can dry out a surrounding soil completely. The cost of removing and replacing poor backfills is high, especially on paved roads. Electrical demand generally increases with time.
Thus, thermal conductivity of a trench backfill material is key aspect of a fully effective buried electrical system that factors into the design of such systems. Design ampacity of a planned electrical system can be limited by the thermal resistivity a backfill material, and the quality of the backfill has a direct bearing on the life and performance of a buried cable.
There are various designs specified for backfilling a trench containing electric cables. These depend on thermal and other properties of the surrounding soils, loading at trench surface, desired cable ampacity, cable type, arrangement of cables, etc. Many of these trench designs specify using specific backfill materials, alone or in combination. These backfill materials include thermal concrete, Fluidized Thermal Backfill (FTB™), and controlled density fill (CDF). Other materials includes gravels and native soils.
Many of these electrical cables are installed within the roadway right-of-way, and thus it is advantageous that these backfill materials harden quickly enough to pave over within a short time of installation so that steel plates do not have to be used, and so that the public is able to safely use the roadway with the least disruption possible.